Mobile communications systems, and in particular Global Mobile Communications Systems employing a constellation of earth orbiting satellites, can exhibit blocking and shadowing of users which are attempting to connect or remain connected to one or more of the satellites. The satellites relay duplex user RF signals to terrestrial-based gateways for further connection to, by example, the Public Switched Telephone Network (PSTN). However, if the access of a given user is blocked to one or more satellites, the user may experience an inability to log onto the system, to initiate or receive calls, or to continue a call that is in progress. This problem is especially apparent when the user is a mobile user, and is in motion with respect to possible RF obstructions, such as foliage and buildings.
With known types of mobile user satellite systems, particularly geosynchronous satellite systems, generally only one satellite is in view of the user. Furthermore, the location of the satellite, on or about the equator, specifies the direction from the user to the satellite. In the northern hemisphere this direction is generally south.
However, recent advances in the communication, computer, and small satellite technology has enabled the concept of providing a constellation of satellites, wherein over large portions of the Earth's surface at least two satellites can be in view of any location. Furthermore, the development of hand-held user terminals using Code Division Multiple Access (CDMA) has made possible multiple satellite coverage employing diversity techniques as a means of mitigating shadowing and blocking of users. By example, a user terminal is enabled to maintain a connection simultaneously through two or more sat ellites of a constellation of LEO satellites that are simultaneously in view.
One problem that is presented when deploying a constellation of, by example, Low Earth Orbit (LEO) satellites is that the user generally cannot directly perceive the best direction to either face or move to in order to maximize his or her ability to establish or maintain communications through one or more of the satellites. This is because the satellites are in motion with respect to the user, and furthermore the satellites may not be, at any given time, located in some predetermined and essentially constant direction with respect to the user (i.e., south as in the geosynchronous satellite case).
For a constellation of earth orbit satellites (in non-polar orbits) it can be shown that the portion of the sky wherein a user will "see" the satellites is a function of latitude. At the equator (0.degree. latitude) the sky is uniformly covered by satellite tracks, whereas at approximately 70.degree. north latitude in only a relatively small portion of the southern sky will a user see a satellite. For intermediate latitudes the size of the region wherein no satellites are found, referred to herein as "obscura", grows progressively small as the equator is approached. The southern latitude case is the mirror image of the northern latitude case for circular orbits.
Furthermore, in a typical case a variety of signal blocking obstructions are found, such as buildings, utility poles, trees, etc., in addition to the region wherein no satellites pass (obscura). It should be apparent that at any given time only a portion of the sky may be optimum for carrying a communication between a terrestrial user terminal and one or more of the satellites.
This problem is compounded if the user is located within a building, as the relatively high frequencies (e.g., S-band) that may be used on the forward link between the satellite and the user terminal will not normally penetrate to any great distance within the building. As a result, the user will typically experience better link quality if the user is located near to a window through which the satellite(s) can be "seen".
It might be thought that if a user would always go to a south-facing window (in the northern hemisphere) that the best link quality will always be experienced. However this may not be the case. By example, assume that the user is being provided with two satellite diversity (i.e., a communication is being simultaneously conveyed through two satellites) and that three satellites are in view. Depending on which two satellites are being used, in one case a south-west facing window may be optimum, while for the second case a south-east facing window may be optimum. In either case a north-facing window would most likely provide for the poorest link quality (absent some fortuitous placement of signal reflecting surfaces).
It is an object of this invention to provide improved methods and apparatus to assist a user of a mobile communication satellite system to log on to, initiate and receive calls, and maintain calls.